A light-water nuclear reactor has a core of nuclear fuel which is cooled by recirculating water. A reactor pressure vessel contains the reactor coolant, which is heated to high temperature by heat produced as a result of nuclear fission produced by the nuclear fuel. Piping circuits carry the heated water or steam to the steam generators or turbines and carry circulated water back to the vessel via feedwater after passing through the main condenser. Operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 288.degree. C. for a boiling water reactor (BWR), and about 15 MPa and 320.degree. C. for a pressurized water reactor (PWR). The materials used in both BWRs and PWRs must withstand various loading, environmental and radiation conditions.
Some of the materials exposed to high-temperature water include carbon steel, alloy steel, stainless steel, and nickel-based, cobalt-based and zirconium-based alloys. Despite careful selection and treatment of these materials for use in water reactors, corrosion occurs on the materials exposed to the high-temperature water. Such corrosion contributes to a variety of problems, e.g., stress corrosion cracking, crevice corrosion, erosion corrosion, sticking of pressure relief valves and buildup of the gamma radiation-emitting Co-60 isotope.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. As used herein, SCC refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other asymmetric metal treatments. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC.
It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 5 ppb or greater. SCC is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals, are produced from radiolytic decomposition of the reactor water. Such oxidizing species increase the electrochemical corrosion potential (ECP) of metals. Electrochemical corrosion is caused by a flow of electrons from anodic to cathodic areas on metallic surfaces. The ECP is a measure of the kinetic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of, e.g., SCC, corrosion fatigue, corrosion film thickening, and general corrosion.
In a BWR, the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2, O.sub.2 and oxidizing and reducing radicals. For steady-state operating conditions, equilibrium concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 are established in both the water which is recirculated and the steam going to the turbine. This concentration of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 is oxidizing and results in conditions that can promote inter-granular stress corrosion cracking (IGSCC) of susceptible materials of construction.
IGSCC proceeds at an accelerated rate in systems in which the ECP is above the critical potential, and at a substantially lower or zero rate in systems in which the ECP is below the critical potential. As used herein, the term "critical potential" means a corrosion potential at or below a range of values of about -230 to -300 mV based on the standard hydrogen electrode (SHE) scale. Water containing oxidizing species such as oxygen increases the ECP of metals exposed to the water above the critical potential, whereas water with little or no oxidizing species present results in an ECP below the critical potential. Thus, susceptibility to SCC in BWRs is highly influenced by corrosion potential. Reduction of the corrosion potential is the most widely pursued approach for mitigating SCC in existing boiling water reactor power plants.
One method employed to mitigate IGSCC of susceptible material is the application of hydrogen water chemistry (HWC), whereby the oxidizing nature of the BWR environment is modified to a more reducing condition. This effect is achieved by adding hydrogen gas to the reactor feedwater. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species on metal surfaces to reform water, thereby lowering the concentration of dissolved oxidizing species in the water in the vicinity of metal surfaces. The rate of these recombination reactions is dependent on local radiation fields, water flow rates and other variables.
The injected hydrogen reduces the level of oxidizing species in the water, such as dissolved oxygen and hydrogen peroxide, and as a result lowers the ECP of metals in the water. However, factors such as variations in water flow rates, reactor power and the time or intensity of exposure to neutron or gamma radiation result in the production of oxidizing species at different levels in different reactors. Thus, varying amounts of hydrogen have been required to reduce the level of oxidizing species sufficiently to maintain the ECP below the critical potential required for protection from IGSCC in high-temperature water.
It has been shown that IGSCC of Type 304 stainless steel used in BWRs can be mitigated by reducing the ECP of the stainless steel to values below -0.230 V(SHE). However, high hydrogen additions, e.g., of about 1 ppm or greater into the feedwater, that may be required to reduce the ECP below the critical potential, can result in a higher radiation level in the steam-driven turbine section from incorporation of the short-lived N-16 species in the steam. Thus, recent investigations have focused on using minimum levels of hydrogen to achieve the benefits of HWC with minimum increase in the main steam radiation dose rates.
An effective approach to achieve this goal is to either coat or alloy the stainless steel surface with palladium or any other noble metal. As used herein, the term "noble metal" means metals taken from the group consisting of platinum, palladium, osmium, ruthenium, iridium, rhodium, and mixtures thereof. The presence of palladium on the stainless steel surface reduces the hydrogen demand to reach the required IGSCC critical potential of -0.230 V(SHE). Compared to the HWC technique, which employs large hydrogen additions to suppress and recombine oxygen and hydrogen peroxide formed by radiolysis to very low levels (e.g., &lt;2 ppb), the noble metal approach requires only that sufficient hydrogen be present so that, as water is formed on the catalytic surface, all oxygen and hydrogen peroxide are consumed (e.g., 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O). Additionally, lower potentials (generally the thermodynamic minimum) are obtained. Depending on the precise location within a BWR, the hydrogen addition required in the noble metal approach may be reduced by a factor of 5 to 100.
The development of techniques to apply noble metal in situ to all wetted components of a reactor represents a breakthrough in extending the applications of the noble metal technology, since manual application (e.g., by thermal spray or fusion cladding) requires complex tooling, is slow and expensive, and can only coat surfaces to which there is sufficiently good access.
U.S. patent application Ser. No. 08/143,513 discloses a technique to coat or dope oxidized stainless steel surfaces in situ by injecting a noble metal compound in the reactor circuit, which noble metal compound then releases species of the noble metal into high-temperature water. As used herein, the term "species" means atoms, ions and molecules. The compound is injected in situ in the form of a solution or a colloidal suspension. As used herein, the term "solution" means both solutions and colloidal suspensions.
The preferred noble metal compound is Na.sub.2 Pt(OH).sub.6. Another suitable compound for use in the invention is palladium acetylacetonate (Pd(CH.sub.3 COCHCOCH.sub.3).sub.2), an organometallic compound, which undergoes thermal decomposition in high-temperature water, thereby releasing palladium atoms which deposit on oxidized surfaces. Alternatively, palladium nitrate, which releases palladium ions upon ionization in high-temperature water, can be used. As used herein, the term "release" also includes the colloidal formation of noble metal molecules. The concentration of noble metal in the reactor water is preferably in the range of 5 to 100 ppb. Doping occurs when noble metal atoms, ions or molecules (i.e., species) released into the high-temperature water deposit on the oxidized surfaces of the flooded reactor components. Other noble metal compounds of organic, organometallic or inorganic nature, as well as compounds of titanium, zirconium, molybdenum, niobium and tungsten can also be used. As used herein, the term "transition metals" means the group of metals consisting of titanium, zirconium, molybdenum, niobium and tungsten. Other suitable noble metal compounds are K.sub.3 Rh(NO.sub.2).sub.6, Pt(NH.sub.3).sub.4 (NO.sub.2).sub.2 and mixtures of Na.sub.2 Pt(OH).sub.6 and K.sub.3 Rh(NO.sub.2).sub.6.
Following noble metal injection, hydrogen can be injected into the reactor water. As hydrogen is added, the potential of the noble metal-doped oxide film on the stainless steel components is reduced to values which are much more negative than when hydrogen is injected into a BWR having stainless steel components which are not doped with noble metal.
The surfaces inside the reactor which become doped with noble metal as a result of the foregoing treatment have catalytic properties. Once these surfaces are doped with noble metal, their ECPs remain low, i.e. below the threshold potential for IGSCC, e.g. &lt;-0.230 V(SHE), in the presence of low concentrations of dissolved hydrogen. Numerous laboratory experiments have confirmed that doping of surfaces with noble metal prevents crack initiation and mitigates crack growth of the structural materials used in the nuclear reactor.
In the laboratory, noble metal doping is accomplished by injecting a solution of a noble metal compound into high-temperature water in a recirculating flow loop comprising a heated high-pressure vessel 10, e.g., an autoclave (see FIG. 1). The vessel has an inlet 12 and an outlet 14 which are in flow communication with a chamber 16 inside vessel 10. The matrix 18 to be coated or doped with noble metal is placed inside the vessel. The vessel inlet 12 is connected to an outlet at the bottom of a water tank 20 via a recirculation pump 22 and a heat exchanger 24. One or more gas bottles 34 is selectively in fluid communication with water tank 20 for dissolving one or more gases (e.g., hydrogen, oxygen and nitrogen) in the recirculating water to obtain the necessary water chemistry conditions. The vessel 10 has a heater capable of heating the water in chamber 16 to a temperature sufficient to cause thermal decomposition of a noble metal compound which is injected at injection point 26 located downstream of heat exchanger 24 directly into the heated high-pressure vessel 10. The heated water inside chamber 16 is returned to the water tank 20 via heat exchanger 24, a backpressure regulator 28 and a water cleanup system 30. The heat exchanger transfers heat from the hot water exiting vessel 10 to the cold water entering vessel 10, whereby the incoming water is pre-heated. A chemical analysis system 32 may be used to sample the water exiting the vessel 10 and determine the chemical composition of the water by a conventional chemical analysis technique.
The concentration of the noble metal in the feed tank 20 is maintained such that after injection, the diluted water stream will have the desired concentration of the noble metal, generally in the range 10 to 100 ppb. The amount of noble metal entering the autoclave 10 is kept constant by maintaining both the concentration in the feed tank and the injection rate constant.
Attempts to measure the actual concentration of noble metal in the autoclave showed that the noble metal in the water sample was present in both ionic as well as non-ionic form. The non-ionic form is likely the result of the formation of colloids during dilution of the noble metal compound in the high-temperature water inside the autoclave. The formation of colloids also complicates the chemical analysis in that if agglomeration of colloidal particles occurs, the colloidal particles may tend to settle in the sampling bottle. Such particulate settling results in some errors, making analysis of noble metal during process application by conventional analytical means more difficult. Thus, alternative approaches to ensure the proper distribution of noble metal during plant application need to be developed to verify the final process application.